1. Field of the Invention
The present invention relates to a battery capacity measurement apparatus, and in particular, to a battery capacity measurement apparatus, taking a gassing voltage into consideration, wherein a full charge capacity during traveling is estimated from a gassing voltage that changes relative to temperature.
2. Description of the Related Art
Electric vehicles use a battery for traveling, hence it is important to accurately estimate the remaining capacity.
For example, in a battery remaining capacity apparatus disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-278351, a battery controller determines the remaining capacity SOC (also referred to as "dischargeable capacity") of a battery immediately after turning on the ignition from a following expression (1), as shown in FIG. 1, in view of an easy calculation, and displays this: EQU SOC=[(Vn.sup.2 -Ve.sup.2)]/(Vs.sup.2 -Ve.sup.2)].times.100% (1)
wherein
Vn: estimated voltage of a battery PA1 Ve: discharge terminating voltage PA1 Vs: full charge voltage.
The full charge voltage Vs is a voltage related to a new one in which a battery does not deteriorate, and is a voltage when the battery is charged at a temperature of 20.degree. C. so as to be fully charged up to 100%.
Calculation of the aforesaid remaining capacity SOC is performed as described below.
The discharge current from the battery and the terminal voltage are collected in a predetermined number to be averaged on a voltage-current coordinate, and when the predetermined number of the averaged data are collected, a correlation coefficient r of the data is determined.
Next, when the correlation coefficient r shows a strong negative correlation, a regression line (also referred to as "approximation line") of the data is determined from the method of least squares. From the approximation line Y (Y=a.multidot.X+b) and a reference current Io, the estimated voltage Vn of the battery at present is estimated.
Then, using the estimated voltage Vn, the present remaining capacity SOC during traveling is determined from the full charge voltage Vs corresponding to the remaining capacity 100% and the discharge terminating voltage Ve corresponding to the remaining capacity 0%, according to the above-described expression (1), and displayed.
Moreover, in hybrid vehicles comprising an engine and a motor, and driven by either of these, a battery controller has recently been used. During traveling, if the remaining capacity decreases, regenerative electric power from an alternator of the hybrid mechanism (a mechanism having a plurality of power sources comprising an alternator, an engine and the like) is charged to the battery. The remaining capacity during traveling is controlled to 60% to 80% so that the battery controller used in such a hybrid vehicle can charge the regenerative electric power at all times during traveling.
However, in general, batteries have such characteristics that the capacity decreases at low temperature compared to that of at the time of high temperature, and at the time of low temperature, even if the battery is fully charged, the capacity does not reach 100%, and for example, the capacity is 80%.
Moreover, there is a case where even if the battery is fully charged, the capacity does not show 100% due to deterioration of the battery, and the capacity shows, for example, 80%.
That is to say, if a temperature drops, or the battery is deteriorated, the remaining capacity does not show 100%, even if the battery is fully charged, hence overcharge occurs.
At the time of high temperature, on the contrary, the voltage of the battery tends to be high, causing insufficient charge.
That is to say, there is a problem in that it cannot be accurately judged, from the remaining capacity of the battery, how much it is chargeable during traveling.
In particular, with a method of determining the remaining capacity from the above-described expression (1), the remaining capacity actually shows 100%, only when the battery is fully charged at a reference temperature (20.degree. C.), in a non-traveling condition.
Accordingly, to prevent an error from arising during traveling, because of the capacity change of the battery due to the temperature, there is a method in which the full charge capacity in the non-traveling condition is made 100%.
However, even if it is attempted to apply this method to a hybrid vehicle in which regenerative electric power of the alternator is charged to thereby keep the remaining capacity (dischargeable capacity) constant during traveling, the temperature changes even at the time of traveling. Therefore, if the method using the expression (1) is applied to the hybrid vehicle, the determined remaining capacity is not a reliable charge capacity, causing a problem in that overcharge or insufficient charge occurs.
On the other hand, batteries reach a gassing voltage (a voltage at which generation of gasses becomes active), when the charge capacity exceeds 90%.
With the conventional remaining capacity measurement apparatus of a battery, however, there is a problem in that calculation of the remaining capacity and charge are not performed, taking the effects of gassing into consideration.